Devices suitable for this use are well known, one example being that disclosed in Brazilian Patent PI 8903982-3 which describes and illustrates an elongated rectangular piece of plastic in the form of two parallel strips connected to each other along a narrow hinge line around which the strips can be folded over each other. One of the strips is formed along its length with a series of small upstanding locking pins, while the other strip is formed with a corresponding series of sockets for receiving the locking pins in locking relation. The strip with the locking pins is fixed, by means of plastic soldering, along the external border of one side of the mouth of a plastic envelope, with the locking pins perforating both borders of the envelope. To close and seal the envelope one simply has to fold the strip with the sockets around the hinge line and press the two strips together until the locking pins enter the sockets and become held. The envelope is then closed with the hinge line situated along the mouth of the envelope, preventing access thereto.
When the envelope is closed and sealed in this way it is no longer possible to open it without breaking the locking pins which, for preference, have weakened bases. Any unauthorized attempt to open the envelope results in the breakage of one or more locking pins, leaving a clear indication of violation or attempted violation. Normal, or authorised, opening of the envelope is carried out, not at the mouth end because it is difficult, but by cutting open the end opposite to the mouth with scissors, thus leaving the closure device intact for gained by unauthorised persons.
A similar device is described in EP-A-0 323 706 which is an earlier version of the product disclosed in Brazilian patent PI 8903982-3. In that particular case, the hinge line is formed by a series of longitudinally separated transverses bridges between the strips.